Centrifugal fans for commercial applications typically include a fan wheel that is rotated by a motor. The fan wheel includes a generally flat, circular back plate and a plurality of spaced-apart blades arranged near the circumferential edge of the back plate. The blades protrude outwardly from the plane of the back plate. As the back plate is rotated by the motor, the blades rotate about the rotational axis of the fan wheel, which axis is perpendicular to the center of the back plate.
The blades are sized so that there is a central cavity within the fan wheel, the cavity being defined between the radially innermost ends of the blades. The blades are angled so that, as the fan operates, inlet air is drawn into the cavity in a direction generally parallel to the fan wheel rotational axis and forced radially outwardly from the cavity. The fan wheel is contained in a housing that directs the outlet air into the distribution system to which the fan is connected.
A generally frustum-shaped inlet cone is mounted near the cavity of the fan wheel. The cone is shaped to direct ambient air into the fan wheel cavity.
The path of the air that flows to the rotating fan wheel forms a vortex. The vortex exhibits a nonuniform velocity gradient as measured normal to the air flow. The vortex and non-uniform inlet air velocity combine to generate turbulence that produces unwanted noise and vibration during operation of the fan.